Central offices supply typically 48 volts to subscriber lines; when a substantially increased current (as distinguished from leakage current) flows down the subscriber line, the central office considers that a station apparatus connected to the line has gone off-hook. Since subscriber lines are of various lengths (some lines being very long), and thus of various resistances, the central office power supplies typically provides full 48 volts to the lines in order to ensure that sufficient current flows in an off-hook condition which will be detected at the central office.
PBXs on the other hand typically operate with much shorter subscriber lines, and therefore do not expect high resistance in the lines. Thus to save cost, their power supplies often supply voltage much lower than central offices (e.g. 27 volts), with the expectation that the lower voltage is what would often be found at the end of a long subscriber line connected to a central office, off-hook current at this lower voltage would be able to be detected, and the system would operate satisfactorily.
Many telephone sets which provide PBX types of features are designed to work with central offices. They are thus designed to operate using the line voltage supplied from a PBX, i.e. in the 48 volt range. If there are two telephone sets connected to one line and one of them goes off-hook, the other one will sense the lower voltage across the line when the first one is busy and will indicate to the user of the other set that the line is busy. This is illustrated in FIG. 1, in which two telephone sets 1A and 1B are connected to a line 3 which is connected to central office 5. Set 1A is shown off-hook and set 1B indicates BUSY on its display.
However, in the case in which the voltage at the telephone sets is lower in the first place, e.g. 27 volts as supplied from a PBX, both sets could detect the lower voltage even if none is are off-hook, and as a result both indicate that the line is busy, prohibiting their use.
Thus station apparatus such as feature telephone sets, fax machines, modems, etc., which sense voltage levels across the telephone line in order to determine if the line is busy, may not operate correctly if operating from a low voltage power supply circuit, such as that supplied from many PBXs.
There has been no way of knowing, other than by trial and error, which station apparatus would be compatible with the lower voltage lines. Those that worked in a connected up and tested state would be accepted, and those that did not were not used for this purpose. There are a large number of different types and suppliers of such station apparatus, and operability has been typically determined in the field, by set-up on the line to be used. Returning apparatus that does not work, and determining apparatus that does work by trial and error, wastes time, is costly, and is annoying, both to the purchaser and to the supplier.